It has been shown in a number of tissues that certain types of extracellular stimuli cause an interconversion of plasma membrane lipids. Two examples are the rapid hydrolysis of inositol phospholipids and the transmethylation of phosphatidylethanolamine to phosphatidylcholine. The former, known as the "PI effect", is associated with stimulus-coupled secretion reactions which utilize Ca++ as a second messenger. The latter has been associated with the activation of adenylate cyclase. We have demonstrated the PI effect in the retina of Xenopus laevis. Light is the stimulus and the effect is in the horizontal cells. We have also demonstrated the transmethylation reaction in frog photoreceptor cells in vivo, but we have not established a stimulus dependency. We are proposing to study the metabolism of retinal membrane phospholipids, with most of our efforts directed towards investigating the physiological role of the PI effect and the transmethylation reactions. These studies will be carried out on whole retinas from Xenopus, and humans, isolated horizontal cells from goldfish, and a metabolically competent, truncated rod cell (RIS-ROS) preparation from goldfish. In retinas, we will use biochemical techniques to identify the precise inositol phosphatide(s) involved in the PI effect. The neurotransmitter(s) responsible for the intercellular communication between photoreceptor and horizontal cells will be identified. The participation of other retinal neurotransmitter candidates in provoking the PI effect in other retinal neurons will be investigated. We will utilize isolated horizontal cells to study the PI effect at the cellular level and attempt to define its role in metabolic events specifically identified with these neurons (i.e. dopamine-sensitive adenylate cyclase, potential-dependent release of GABA, or Ca++ dependent regenerative voltage responses). In the RIS-ROS, inositol phosphatides and phosphatidic acid will be examined for a role in Ca++ translocation associated with photon capture. The transmethylation reactions will be studied in isolated horizontal cells, and correlated with other known physiological events, such as dopamine-sensitive adenylate cyclase. In RIS-ROS, we will attempt to identify by autoradiography and by subcellular fractionation the intracellular location of the transmethylase enzymes.